Design of calibration‐free RF pulses for T 2$$ {}_2 $$‐weighted single‐slab 3D turbo‐spin‐echo sequences at 7T utilizing parallel transmission

Abstract

AbstractPurposeT‐weighted turbo‐spin‐echo (TSE) sequences are a fundamental technique in brain imaging but suffer from field inhomogeneities at ultra‐high fields. Several methods have been proposed to mitigate the problem, but were limited so far to nonselective three‐dimensional (3D) measurements, making short acquisitions difficult to achieve when targeting very high resolution images, or needed additional calibration procedures, thus complicating their application.MethodsSlab‐selective excitation pulses were designed for flexible placement utilizing the concept of k‐spokes. Phase‐coherent refocusing universal pulses were subsequently optimized with the Gradient Ascent Pulse Engineering algorithm and tested in vivo for improved signal homogeneity.ResultsImplemented within a 3D variable flip angle TSE sequence, these pulses led to a signal‐to‐noise ratio (SNR) improvement ranging from 10% to 30% compared to a two‐dimensional (2D) T2w TSE sequence employing ‐shimmed pulses. field inhomogeneities could be mitigated and artifacts from deviations reduced. The concept of universal pulses was successfully applied.ConclusionWe present a pulse design method which provides a set of calibration‐free universal pulses (UPs) for slab‐selective excitation and phase‐coherent refocusing in slab‐selective TSE sequences.